Bariatric procedures offer sustained and significant weight reduction with the potential to effect general patient health, comorbid conditions, quality of life and the healthcare system. A Bariatric Surgery Clinical Research Consortium (BSCRC) wilt provide important prospective information about the true impact of the procedure on patients and opportunities to better explore the physiologic mechanisms that result in post-surgical weight loss. The BSCRC will prospectively collect clinical, demographic, epidemiological, laboratory and histological information. In addition to this database the BSCRC will complete the following studies: 1. A cross-sectional examination of the epidemiology of non-alcoholic steatohepatitis (NASH) in patients undergoing bariatric surgery and a prospective evaluation of the effect of surgically induced weight loss on the severity NASH and cellular markers of cytotoxic activity. There is a need for epidemiologic information about the prevalence, risk factors for, and impact of fatty liver disease in patients undergoing bariatric surgery. We propose a study to evaluate liver histology in a large group of patients undergoing bariatric procedures. Patients with evidence of NASH by biopsy will undergo subsequent liver biopsy at one year to determine if NASH improves with rapid weight loss. Patients with NASH who improve after weight loss represent an important model for evaluating the cellular mechanisms that are involved in the development of NASH. This study will evaluate markers of oxidative stress and hepatic mitochondrial structure to determine their relationship to NASH during and after rapid, surgical weight loss. 2. A prospective evaluation of the relationship between ghrelin, PYY3-36, appetite and weight loss outcomes after gastric bypass. Ghrelin and PYY3-36 are gut-derived peptides that are involved in energy homeostasis principally through their effect on appetite. We propose a prospective study to determine the relationship of ghrelin and weight loss after gastric bypass and to determine if the degree of ghrelin suppression is correlated to hunger suppression and/or the amount of weight lost. This study will determine if inclusion of ghrelin producing cells in the gastric pouch is correlated to worsened weight loss outcomes. Lastly the study will begin to evaluate the relationship of PYY3-36 and ghrelin and determine if it is altered by or in response to the physiologic changes associated with gastric bypass.